Although biphenyl based liquid crystal materials have been utilized for some time in optical displays due in part to relatively low operating power consumption, such materials generally have been unable to provide attractively low viscosities in conjunction with fast response times particularly at low temperatures to controlled electrical fields over a broad nematic operating temperature range.
Generally it has been the practice in the past to provide mixtures of two or more biphenyl based liquid crystalline materials to provide lower viscosity and a broader nematic operating temperature range than is ordinarily associated with just one of the biphenyl based liquid crystalline materials. In addition, lower viscosity and broader nematic operating temperature ranges particularly in lower temperature regions have been found to be characteristic where intermediate phenyl groups of liquid crystalline materials have been appended with methane or chlorine groups. Although such mixtures of biphenyl type liquid crystals or liquid crystals featuring methane or chlorine groups appended to a phenyl group may be used to advantage, they have been unable to achieve a fast response time to electrical fields at lower nematic operating temperatures and typically have been unable to achieve a response time of less than one minute at -30.degree. C.
An example of liquid crystalline materials having methane or chlorine groups appended to an intermediate phenyl group dependent upon the nature of phenyl groups chemically linked to the intermediate is proposed in U.S. Pat. No. 4,082,686. Although the liquid crystalline materials disclosed are proported to remain nematic over a wide temperature range, they are based upon Shiff Bases that are known to be unstable and generally exhibit a mesomorphic range higher than about 45.degree. C. and do not exhibit a mesomorphic temperature lower than about -20.degree. C. even when mixed with other designated liquid crystalline materials and typically exhibit slow response times to an electrical field in the lower nematic operating temperature range.
An example of mixing certain nematic liquid crystalline materials in specific proportions in order to achieve a wide nematic operating temperature range is proposed in U.S. Pat. No. 4,090,975. Although the mixture may be used to advantage, the process requires the costly preparation and storage of chemically unstable Schiff's base nematic liquid crystalline compounds that are apt to deteriorate if not used within a short time after their preparation.
An example of the use of a cyclohexane ring in nematic liquid crystalline materials is proposed in U.S. Pat. No. 4,202,791. Disclosed is a typical alkylthioester type material in which one of the phenyl groups has been replaced with a 4 substituted cyclohexane derivative. Although the materials are proported to be useful singularly or in mixtures with at least one other nematic liquid crystalline and/or homologous non-liquid crystalline compounds they typically have been unable to possess fast response times to an electrical field at their lower nematic operating temperatures and more typically have been unable to exhibit a response time of less than one minute at -30.degree. C.
Another example of a biphenyl type liquid crystal having a chlorine atom appended to one of the phenyl groups as a means of broadening the nematic operating temperature range is proposed in U.S. Pat. No. 4,216,109. Although the materials disclosed are proported to have a nematic operating temperature range of about 33.degree. C. to about 104.degree. C., mixing equal moles of selected mixtures is reportedly able to broaden the range to include only from about -5.degree. C. to about 100.degree. C., they generally exhibit slow response times at their lower nematic operating temperatures that are typically in the order of greater than one minute.
Another example of incorporating a cyclohexane ring in liquid crystal compositions is disclosed in U.S. Pat. No. 4,228,030. The requirement is imposed however to inventory, store and prepare a mixture comprising specific liquid crystalline materials incorporating a cyclohexane ring with a triphenyl composition that, although, chemically stable, exhibit a nematic operating temperature range of at most 0.degree. C. to about at least 85.degree. C. and have poor response times to an electrical field in the lower nematic temperature range typically in the order of 10 seconds or more at -15.degree. C.
An example of liquid crystalline materials incorporating disubstituted cyclohexane groups having either two cyclohexane groups or two phenyl groups or a phenyl and a cyclohexane group appended thereto is disclosed in U.S. Pat. No. 4,229,315. However, even by mixing variations of the prescribed materials in specific amounts, the nematic operating temperature range at best is still generally between about -10.degree. C. and about 85.degree. C. with slow response times to an electrical field that are typically greater than one minute at their lower nematic operating temperatures.